EP0506527B1 - Device and method for testing an array antenna during operation - Google Patents

Description

The present invention relates to a method and a device for on-line testing, that is to say in operation of an antenna composed of a plurality of radiating sources.

A particular field of application of the invention relates to secondary radar antennas of the monopulse type, making it possible to radiate in space the pulses produced by an IFF (Identification friend or foe) interrogator, that is to say friend identification enemy) and to capture any responses from the aircraft questioned. Such antennas consist of a plurality of radiating elements, called columns. On site, it is very difficult to identify if one or more of these columns are broken down, and, in the event of a drop in antenna performance, we do not know how to precisely locate the column or columns which are the cause.

A known practical method of locating faulty columns consists in placing a detector successively in front of each column to check whether their radiation is indeed that expected. The drawback of such a method lies in the interaction of the radiation diagrams of the columns neighboring the column tested, which makes the measurements made imprecise. In addition, this method requires the temporary stopping of the antenna in its interrogation and reception of aircraft response functions.

• le niveau moyen des lobes secondaires peut permettre de détecter une panne. En fait, cette méthode est écartée car elle ne permet pas d'extraire une information fiable concernant les colonnes d'extrémités pour lesquelles les variations du niveau moyen sur les lobes secondaires sont trop faibles ;
• la fonction caractéristique de l'alignement des dipôles s'annule autant de fois qu'il y a d'éléments rayonnants dans l'alignement. L'observation du nombre de zéros devrait donc permettre la détection de pannes. Cependant, c'est une méthode peu fiable en ce sens qu'en pratique, des réflexions sur obstacles peuvent faire disparaître des zéros ;
• le diagramme de rayonnement d'une antenne se déduit de la Transformée de Fourier de l'éclairement de cette antenne. En calculant la Transformée de Fourier inverse, on devrait donc pouvoir reconstituer l'aspect de l'éclairement de l'antenne, et détecter ainsi des trous à l'endroit des colonnes en panne. En fait, cette méthode n'est pas exploitable car on ne peut pas récupérer l'information essentielle du signal, à savoir sa phase.

There are also different methods working directly on the antenna radiation pattern and based on the following principles:

• the average level of the secondary lobes can make it possible to detect a failure. In fact, this method is discarded because it does not allow to extract reliable information concerning the end columns for which the variations of the level medium on the side lobes are too weak;
• the characteristic function of the alignment of the dipoles is canceled as many times as there are radiating elements in the alignment. Observing the number of zeros should therefore allow the detection of faults. However, it is an unreliable method in the sense that in practice, reflections on obstacles can make zeros disappear;
• the radiation pattern of an antenna is deduced from the Fourier Transform from the illumination of this antenna. By calculating the inverse Fourier Transform, we should therefore be able to reconstruct the aspect of the illumination of the antenna, and thus detect holes at the location of the faulty columns. In fact, this method is not usable because we cannot recover the essential information of the signal, namely its phase.

The inverse Fourier transform is p. ex. used in the document IEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, vol. 36, No. 6, June 1988, pages 884-889, New York, US, J.J. LEE et al. : "Near-Field Probe Used as a Diagnostic Tool to Locate Defective Elements in an Array Antenna".

The object of the invention is to overcome the above drawbacks by proposing a method and a device for on-site testing of the operation of a multi-source antenna which makes it possible to precisely locate the radiating source or sources which are faulty, without the primary function of this antenna is hampered.

Although the description of the method and the device according to the invention is made in the context of the application to a secondary radar antenna, it is already important to note that the invention is not limited to this application particular, as we will see later.

• constituer une bibliothèque de diagrammes de rayonnement de l'antenne d'une part, lorsque toutes les sources rayonnantes fonctionnent normalement et d'autre part, lorsqu'au moins l'une desdites sources est mise volontairement en panne ;
• relever le diagramme de rayonnement de l'antenne lors de son fonctionnement sur site ;
• calculer le coefficient de corrélation dudit diagramme de rayonnement avec chacun des diagrammes de rayonnement contenus dans ladite bibliothèque ;
• calculer la valeur maximale du coefficient de corrélation, de façon à détecter une panne éventuelle d'une ou de plusieurs desdites sources rayonnantes.

The subject of the invention is more precisely a method for testing on site an antenna comprising a plurality of radiating sources, characterized in that it consists in:

• build up a library of antenna radiation patterns on the one hand, when all the radiating sources are operating normally and on the other hand, when at least one of said sources is voluntarily broken down;
• take note of the antenna radiation pattern when on-site operation;
• calculating the correlation coefficient of said radiation pattern with each of the radiation patterns contained in said library;
• calculating the maximum value of the correlation coefficient, so as to detect a possible failure of one or more of said radiating sources.

• des premiers moyens de mesures et de mémorisation dans une bibliothèque des diagrammes de rayonnement de l'antenne d'une part, lorsque toutes les sources rayonnantes fonctionnent, et d'autre part, lorsqu'au moins l'une desdites sources est mise volontairement en panne ;
• des seconds moyens de mesure du diagramme de rayonnement de l'antenne lors de son fonctionnement sur site ;
• des moyens de calcul du coefficient de corrélation du diagramme de rayonnement de l'antenne et de chaque diagramme contenu dans ladite bibliothèque ;
• des moyens de calcul de la valeur maximale du coefficient de corrélation pour détecter une panne éventuelle.

The invention also relates to an on-site test device for an antenna comprising a plurality of radiating sources, characterized in that it comprises:

• first means of measurement and storage in a library of antenna radiation diagrams on the one hand, when all the radiating sources are working, and on the other hand, when at least one of said sources is voluntarily activated breakdown ;
• second means for measuring the radiation pattern of the antenna during its operation on site;
• means for calculating the correlation coefficient of the antenna radiation diagram and of each diagram contained in said library;
• means for calculating the maximum value of the correlation coefficient to detect a possible failure.

• la figure 1 est une représentation de la face avant d'une antenne de radar secondaire ;
• la figure 2 est une représentation de la face arrière de cette même antenne ;
• la figure 3 représente les trois diagrammes de rayonnement de l'antenne précédente ;
• la figure 4 est une structure de blocs représentative des étapes à effectuer pour le test selon l'invention.

The test method and device will be better understood during the description which follows, relating to the following figures:

• Figure 1 is a representation of the front face of a secondary radar antenna;
• Figure 2 is a representation of the rear face of the same antenna;
• Figure 3 shows the three radiation patterns of the previous antenna;
• FIG. 4 is a block structure representative of the steps to be carried out for the test according to the invention.

Figures 1 and 2 respectively represent the front face and the rear face of a monopulse type secondary radar antenna. This antenna comprises a beam 1 on which are fixed radiating columns (2, 3), for example thirty six in number, the thirty five columns 2 being placed on the front of the antenna, and the column 3 being placed at the back. Radiation can be obtained for example by a plurality of radiating dipoles placed on each column. The antenna is intended on the one hand, to radiate in space the signals produced by an interrogator (not shown) and on the other hand, to receive the responses emanating from the transponders mounted on the planes. For this purpose, it has three radiation diagrams shown diagrammatically in FIG. 3, obtained by different distributions, in amplitude and in phase, of the energy between the thirty six radiating columns. The antenna first radiates a diagram (Σ) 3 presenting a directive main lobe 4 surrounded by secondary lobes 5, by means of which it emits a pair of interrogation pulses. To prevent a transponder on board a nearby aircraft from responding to the pulses emitted by the secondary lobes 5, a third pulse interspersed with the previous ones is emitted according to the almost omnidirectional radiation diagram (Ω) 6, which completely covers the secondary lobes 5 Thus, only the aircraft located in the coverage area of the main lobe 4 are required to respond to the interrogations of the antenna. The last diagram 7, or diagram (Δ), makes it possible to refine the position of the airplane which emits a response.

FIG. 4 explains the different stages of the method according to the invention.

A first step referenced 11 consists in storing in a library the three diagrams Σ _i , Ω _i and Δ _i measured for example on the antenna site (i representing the column index) in the particular configuration where column i is voluntarily broken down. The index 0 corresponds to the case where no column is broken.

The need to use the triplet (Σ, Δ, Ω) of the radiation diagrams is justified by the fact that a single diagram is not sufficient for the precise location of a faulty column. Indeed, the energy applied to the input of the channel (Σ) of the antenna is distributed between the thirty five front columns with the same phase and decreasing amplitudes from the center of the antenna towards the ends. Thus, the diagram (Σ) makes it possible to detect only the failures of the columns close to the center of the antenna, with moreover an ambiguity on the relative position of these columns with respect to this center. For the radiation diagram (Δ), the seventeen columns located on the left of the central column are supplied in phase opposition with respect to the seventeen columns located on the right. This removes the previous ambiguity. In addition, the amplitude distribution of the Δ diagram makes it possible to treat more specifically the columns placed on either side of the center of each half of the antenna. Finally, the diagram (Ω), for which only the rear central column is supplied in phase opposition with respect to the thirty-five other columns, makes it possible to detect the failure of this rear column.

The second step of the method according to the invention, referenced 22, consists in measuring on site at predetermined times the three radiation patterns Σ, Ω, Δ of the antenna during its operation under IFF interrogation.

The third step noted 44 is the calculation of the correlation coefficient for each pair of signals (Σ _i , Σ), (Δ _i , Δ), (Ω _i , Ω), knowing that this correlation coefficient for a pair of signals (x _i (t), (x (t)) is given by the relation: $$\text{ρ}{}_{{\text{x}}_{\text{i}}}\text{=}\frac{{\text{x}}_{\text{i}}\text{(t) x (t)}}{\sqrt{\overline{{\text{x}}_{\text{i}}^{\text{2}}\text{(t)}}\text{.}\overline{\text{x² (t)}}}}$$

This coefficient ρ _{x i} allows to assess the resemblance of the two signals x (t) and x _i (t), knowing that the maximum resemblance is theoretically reached for ρ _{x i} = 1, and the total decorrelation for ρ _{x i} = 0.

Steps 44 and 55 thus allow the detection of column i which has failed. More precisely, step 44 makes it possible to calculate the maximum values of the coefficients ρ _{Σ i} , ρ _{Δ i} and ρ _{Ω i} ; these values are not necessarily 1 in practice because the resemblance is never perfect. Column i identified as probably broken down by step 55 corresponds to the highest correlation coefficient. In the case where several columns are faulty, the operation is repeated from step 33 in order to examine all of the columns. Instead of iterating, we could just as easily design a larger library than the previous one, containing fault configurations on more than one column.

The invention also provides a device for implementing the method as described above.

. On peut obtenir des diagrammes de rayonnement très précis en relevant les amplitudes crêtes des réponses de la balise pour une position de l'antenne variant de 0° à 360° par un pas qui se rapproche le plus possible de la résolution du codeur. L'obtention de tous les points nécessite de faire les mesures sur plusieurs tours d'antenne. Cependant, il ne s'agit pas d'une contrainte car l'antenne continue d'assurer sa fonction d'interrogateur IFF.Steps 11 and 22 of the process are readings of radiation diagrams on site. A beacon on the ground placed within the optical range of the radar (typically between 1 km and 50 km) is used to make this beacon functioning as an aircraft transponder, except that it responds to all the interrogations emitted by the 'antenna. The three diagrams (Σ), (Δ) and (Ω) are read by measuring the peak amplitudes of the three pulses constituting the response of the tag. This reading is done over several antenna turns, which makes it possible to obtain very precise radiation patterns. The idea is based on the fact that the position of the antenna is generally known thanks to an optical encoder for example, comprising n bits, and whose resolution is $$\frac{\text{360 °}}{{\text{2}}^{\text{not}}}$$

. Very precise radiation patterns can be obtained by reading the peak amplitudes of the beacon responses for a position of the antenna varying from 0 ° to 360 ° by a step which is as close as possible to the resolution of the encoder. Obtaining all the points requires making the measurements over several antenna turns. However, this is not a constraint because the antenna continues to perform its function of IFF interrogator.

This device thus makes it possible to measure very precisely on the one hand, the triplets (Σ _i , Δ _i , Ω _i ) which must therefore be stored in a library, and on the other hand, the triplet (Σ, Δ, Ω ) in order to detect one or more possible faults. The rest of the test device can be carried out by any means allowing the calculation of the three correlation coefficients ρ _{calculer i} , ρ _{Δ i} and ρ _{Ω i} , to search for their maximum, and thus to determine the column probably broken.

As we have already pointed out previously, the invention is absolutely not limited to antennas of secondary radars of the monopulse type and can be extended to all cases of antennas having at least two radiating sources.

In addition, the constitution of the library can also be done by simple mathematical calculation.

Claims (9)

1. Method for on-site test of an antenna including a plurality of radiating sources, characterized in that it consists in:

   - compiling a library of radiation patterns of the antenna on the one hand, when all the radiating sources are operating normally and, on the other hand, when at least one of the said sources is deliberately made unserviceable (11);

   - plotting the radiation pattern of the antenna when it is operating on site (22);

   - calculating the coefficient of correlation of the said radiation pattern with each of the radiation patterns contained in the said library (33);

   - calculating the maximum value of the coefficient of correlation (44), so as to detect a possible failure of one or more of the said radiating sources (55).
2. Test method according to Claim 1, characterized in that the library includes the radiation patterns of the antenna when each of the radiating sources is made unserviceable in turn.
3. Test method according to one of the preceding claims, characterized in that the radiation patterns contained in the said library are plotted on site.
4. Test method according to any one of the preceding claims, characterized in that the said antenna is a monopulse-type secondary radar antenna, and in that its operation is represented by a triplet of radiation patterns.
5. Test method according to Claim 4, characterized in that the said triplet consists of a sum (Σ) pattern, of a difference (Δ) pattern and of a quasi-omnidirectional (Ω) pattern.
6. Test method according to any one of Claims 4 and 5, characterized in that the calculation of the coefficient of the correlation is done for each radiation pattern constituting the said triplet.
7. Device for on-site test of an antenna consisting of a plurality of radiating sources, characterized in that it includes:

   - first means for measurement and memory storage in a library of the radiation patterns of the antenna on the one hand, when all the radiating sources are operating, and, on the other hand, when at least one of the said sources is deliberately made unserviceable;

   - second means for measuring the radiation pattern of the antenna when it is operating on site;

   - means of calculating the coefficient of correlation of the radiation pattern of the antenna and of each pattern contained in the said library;

   - means of calculating the maximum value of the coefficient of correlation in order to detect a possible failure.
8. Test device according to Claim 7, characterized in that the said first means perform the radiation pattern measurements on site.
9. Test device according to any one of Claims 7 or 8, characterized in that the said antenna is a monopulse-type secondary radar antenna and in that the said means for on-site measurement of the radiation patterns consist of a ground beacon placed within the optical range of the radar, and responding to all the interrogations from the antenna, and of means for measuring the peak amplitude of the responses from the said beacon over several antenna revolutions.

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